tabled_derive/ Fix issue with Cow usage

[zhiburt]

Seems to work @elenakrittik?

tabled/tabled/tests/derive/derive_test.rs

Lines 1084 to 1091 in 7ec7973

	#[test]
	fn test_display_with_2() {
	#[derive(tabled::Tabled)]
	struct Struct<'a> {
	#[tabled(display_with("std::path::Path::display"))]
	path: &'a std::path::Path,
	}
	}

close #395

[elenakrittik]

Yep, works for me with this patch. Thanks